This invention relates generally to a method and apparatus for improving filtration efficiency. More specifically, this invention relates to a method and apparatus for back pulsing membrane filters to remove the concentration of particulate matter on the filtration surface of such filters.
The need for cleaning the surface of a membrane filter upon which has been accumulated particular matter of various types contained within the fluid to be filtered has been long recognized in the filtration art. This technique is known as "backwashing". For example, in filtration applications that involve backwashing with the filtrate at given intervals, it has been proposed to use two supply means (e.g. pumps) as shown in the representation of FIG. 1. With two-way valves V1 and V2 kept open and a two-way valve V3 closed, a feed material to be filtered is supplied by a supply means such as pump P1 for filtration. The filtration membrane divides the feed into two treated streams, which are conducted via a concentrated solution outlet line "a" and a recovered filtrate line "b".
The filtrate is diverted back at given intervals to the filtration membrane for cleaning or "backwashing". The backwashing is accomplished when necessary for a given time period in the manner known to those of skill in the art, e.g., by closing the two-way valves V1 and V2 and opening the valve V3 and then forcing the filtrate by a second supply means P2 through a backflow supply line "c" and the two-way valve V3. In this connection, Japanese Patent Application Public Disclosure No. 256425/1992 reveals an example of filtration where the supply means P2 uses a compressed gas.
It has been determined that in the filtration process frequent backwashing with a small fluid supply is more effective than less frequent backwashing with a larger supply. One method of performing such frequent flow reversing is to use a centrifugal pump and to open and close valves at short intervals. However, presently there is no centrifugal pump commercially available which is suitable for small-scale operations. If manufactured specifically for such an operation, the pump would not be acceptable because it usually involves difficulties in flow rate control.
On the other hand, while use of a positive displacement pump is desirable in that it permits control of flow rate with ease and is commercially available for small-scale applications, frequent, intermittent running of such a pump is not advisable from the viewpoint of maintenance. Therefore, two pumps are usually combined with bypass lines, and solenoid-operated valves are intermittently opened and closed in response to signals from a timer, thereby performing intermittent flow reversing. However, continuous operation of two pumps is undesirable because both pumps can be simultaneously degraded in performance from wear or other reasons.
One way of overcoming the foregoing difficulties of the prior art is to adopt pneumatic equipment which drives the liquid. The tank for receiving the liquid for flow reversing is required to be pressure-resistant, and valves are indispensable for pressure application and evacuation. An air compressor is also necessary. These components make the noise reduction of the equipment difficult. In addition, frequent fixed-volume flow reversing can hardly be attained because the gas has greater compressibility than the liquid.
Thus the afore-described filtration systems of the prior art require two pumps, or a pressure vessel and pneumatic equipment or the like and their systematic interconnection to realize the flow reversing necessary for backwashing. For these reasons the overall volume or floor space the whole equipment occupies is significant, and this configuration is not efficient and compact in design or construction. The discovery of a more desirable method and the concomitant development of equipment that will simplify the filtration system to accommodate backwashing would be highly desirable.